LATE CENOZOIC TECTONICS OF THE NORTHERN ITOIGAWA-SHIZUOKA TECTONIC LINE, CENTRAL JAPAN

The Itoigawa-Shizuoka Tectonic Line (ISTL) is regarded as an onshore plate boundary between the North American and the Eurasian Plates. Because of lacking of geologic and geomorphic evidence indicative of distinct fault history for past several millions years, late Cenozoic plate motion and associated tectonics are still unclear. This study reconstructed geomorphic and geologic structural evolution to discuss late Cenozoic fault history, based on the chronology of terrestrial sequence, landslide geomorphology and folding structure surrounding to the northern ISTL. The northern ISTL is composed of the geologic Otari-Nakayama fault and the active fault system, bounding the Omine area on the east and west, respectively. Late Pliocene to Early Pleistocene fluvial and volcaniclastic sequence crop out in the central Omine area. At 1.6 or 1.5 Ma, east-flowing rivers formed the Aokidaira Member of Omine Formation that unconformably overlies the lower Pleistocene welded tuff bed and the Otari-Nakayama fault. The congromeratic Iyari Formation was deposited along the active faults at 0.9 or 0.8 Ma. Scarp development of the active faults then resulted in the disappearance of east-flowing rivers across the central Omine zone. The Middle to Late Pleistocene Shingyo Formation is a landslide-related sediment on the upthrown side of active faults. The late Middle Pleistocene large-scale landslides also developed there. Small-scale folding structure related to the Otari-Nakayama fault was formed after the normal Olduvai Subchron in view of paleomagnetism of volcanic successions. The geologic and geomorphic evidence indicates following tectonics of the northern ISTL. The Otari-Nakayama fault had been active from Early or Middle Pliocene to early Early Pleistocene at the earliest. Activity of the northern ISTL advanced basinward from the Otari-Nakayama fault to the active faults immediately before 1.6 or 1.5 Ma. The active faults has been active and progressively developed its scarp since then. This thrust front migration suggests the development of northern ISTL through a Pliocene and Quaternary E-W transpressive regime, and constant westerly relative motion of the North American against the Eurasian Plates.